No implementation of Pages is a Word clone, and does not use field variables for additional insert menu items. LibreOffice Writer offers field codes because it is designed as a Word replacement.
In Pages '09, inserted document dates will auto-update each time that the document is opened. In any release of Pages v5, dates do not auto-update. You would have to click the date in the document, and then select Set to Today. An alternative would be to disable the footer, and replace it with a one-row table using the now() function in a table cell. This function automatically updates the date each time the document is opened. You may find it difficult to eliminate the gap that results from right-aligning a filename in one cell, and left-align now() in an adjacent cell, then merge both cells.
As this is a user-to-user community, suggesting desired features in Pages is not seen by the Pages product team. If that is your goal, use the Pages menu : Provide Pages Feedback form.